This application is for the competitive renewal of ROl A143847 entitled "Structure/function relationships in HIV-1 co-receptors." Gp120 binding to the CD4 receptor drives reordering of the gpl2O core structure and creates/exposes a co-receptor binding site on gpl2O. CCR5 and CXCR4 are the most biologically relevant HIV-1 co-receptors. Over the past two and a half years, grant A143847 has supported a number of studies that enabled my laboratory to map the determinants of CCR5 and CXCR4 co-receptor function. We demonstrated that negatively charged residues and sulfotyrosines in the CCR5 amino-terminal domain (Nt) interact directly with gpl2O and are indispensable for HIV-1 fusion and entry. We also demonstrated that CXCR4 co-receptor function depends on similar residues in the Nt and second extracellular loop. Our studies with inhibitors of CCR5 co-receptor function showed that regions not directly involved in gpl20 binding also play an important role in viral entry. These findings successfully address the original specific aims and lay the groundwork for advanced studies of co-receptor structure/function relationships. Our continuing goal is to define how HIV-1 envelope glycoproteins interact with fusion co-receptors. To this end, we will elucidate the role of sulfotyrosines in CCR5- and CXCR4-mediated fusion and entry, study co-receptor interactions with envelope glycoproteins from non-clade B isolates, and characterize novel lead compounds that inhibit viral entry by interfering with CCR5 or CXCR4 co-receptor function. Our work will provide a detailed molecular picture of the protein complex that mediates HIV-1 membrane fusion and viral entry, and will advance the development of more potent and specific inhibitors that are clinically relevant.